In the food processing industry, large quantities of water are utilized to wash, clean and move processing debris from agricultural products as they are prepared for consumption. Waste water is commonly disposed of in two ways, by recycling and reusing the water, or by disposing of the water by applying it as irrigation water to crop land.
Water may be recycled where water is used to separate processing debris from a desired food product. In these cases, if the water is cleaned of debris, it may be recycled to pick up additional debris from processed plant materials. Wash water is often used in a counter-flow arrangement. Fresh water is used to perform the final rinse on an agricultural commodity whereupon it is filtered and reused for intermediate washing steps. Finally, the wash water is utilized for the initial or first wash, whereupon, after filtering, it is pumped to an irrigation system for application to the fields.
After each use of the water, it is imperative that as much debris be removed as quickly and simply as possible from the waste water stream so that it may be recycled multiple times and the total wash water kept to a minimum. Further, in order to prevent the clogging of the irrigation system used to apply the waste water to agricultural land, it is imperative that the suspended materials above a given size be removed from the waste water before it enters the irrigation piping system.
The optimal waste water treatment system should pass large amounts of water while at the same time removing all or nearly all of the suspended material above a given size. Two problems complicate the design of waste water screens. One is the removal of the residue from the screen. The other is clogging of the screen with the residue. A solution to the second problem has generally been found in the employment of wedge wire as opposed to ordinary wire in the manufacture of screens. Wedge wire, as the name implies, is wire with a wedge-like cross-section of a truncated triangle. The bases of the triangles form the filtering surface. Thus any waste which passes the exterior surface of the wedge wire screen experiences increasing clearance between the adjacent sides of the wedge wires, thus passing freely through the screen. A conventional screen employing round wires has a minimum clearance in the middle of the screen. As the round wires form outwardly-facing and inwardly-facing wedged openings, the outwardly-facing openings tend to collect waste material which becomes wedged therebetween.
For the removal of the residue or waste solids, a number of techniques have been employed. One typical technique is to form the filter screen into a drum. Spaced within the drum is a waste water feed employing overflowing weirs which present the waste water to the screen as a narrow curtain of water parallel to the axis of rotation of the drum. Rotation of the drums serves two purposes. First, a constantly fresh section of the screen is presented to the stream of waste water. Second, the rotational action of the drum causes the solids or waste to tumble and agglomerate. The agglomerated material may then be moved by a series of diverters or a single, continuous diverter which is arranged in a spiral pattern and continuously moves solids to the drum end for removal. The smooth flow of water onto the screen surfaces is highly desirable. However, turbulent flows or agitated water flows of conventional filters hamper this end.
The natural distribution of waste particles in wash water includes smaller particles, which tend to be more numerous and larger particles which generally contain the bulk of the material entrained in the waste water. It has been found that the overall speed of water filtration can be improved by using two filters in sequence, first a coarse filter which removes the majority of the volume of suspended material, and then a fine filter which removes the more numerous but finer particles. The use of two filters, however, has the undesirable effect of doubling the amount of equipment required.
What is needed is a waste water filter which effectively filters water with minimal clogging and with two progressive filters in a single machine.